Graded step waveguide twist

ABSTRACT

A waveguide twist is constructed of basic sections arranged to form a continuous waveguide which rotates the plane of polarization of the guided waves through a large angle in a short distance without appreciable wave energy reflection. Each basic section is approximately one quarter wavelength long and has portions of its broad walls sloped to cause the plane of polarization of the wave energy propagating through the waveguide to be gradually rotated without storage of an appreciable amount of energy in the section and without encountering any abrupt change in cross-sectional area within the waveguide twist. The sloped wall portions form graded steps which replace the abrupt steps that occur at the transitions between segments of a shear twist and &#39;&#39;&#39;&#39;capacitive&#39;&#39;&#39;&#39; recesses in the basic section are provided to tune out the shunt inductance of the section.

United States Patent 51 Mar. 21, 1972 Riblet [5'41 GRADED STEP WAVEGUIDE TWIST [72] Inventor: Henry J. Riblet, 35 Edmunds Road, Wellesley, Mass. 02181 [22] Filed: July 17, 1970 [211 App]. No.: 55,857

[52] 11.8. C1 ..333/98 11, 333/21 A, 333/34 [51] Int. Cl ..1101p 1102, 1-101p 11/00 [58] Field 01 Search ..333/98 R, 21 A, 34

{56] References Cited UNITED STATES PATENTS 2,540,839 2/1951 Southworth ..333/98 R X 2,736,867 2/1956 Montgomery... .....333/98 R X 2,968,771 l/l9 6l De Loach, Jr.... .....333/98 R X 3,046,507 7/1962 Jones, Jr. ..333/95 R Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorney-Louis Orenbuch [57] ABSTRACT A waveguide twist is constructed of basic sections arranged to form a continuous waveguide which rotates the plane of polarization of the guided waves through a large angle in a short distance without appreciable wave energy reflection. Each basic section is approximately one quarter wavelength long and has portions of its broad walls sloped to cause the plane of polarization of the wave energy propagating through the waveguide to be gradually rotated without storage of an appreciable amount of energy in the section and without encountering any abrupt change in cross-sectional area within the waveguide twist. The sloped wall portions form graded steps which replace the abrupt steps that occur at the transitions between segments of a shear twist and capacitive" recesses in the basic section are provided to tune out the shunt inductance of the section.

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02 HG6F E FWCi 6(3 INVENTOR HENRY J FUBLET BY O LuLW 6:&L4v@%bbA ATTORNEY GRADED STEP WAVEGUIDE TWIST FIELD OF THE INVENTION This invention relates in general to the guided transmission of electromagnetic waves and more particularly relates to waveguide twist devices for rotating the plane of polarization of guided electromagnetic waves.

DISCUSSION OF THE PRIOR ART In the transmission of electromagnetic waves through waveguides, it is sometimes necessary to rotate the plane of polarization of the waves. In the case of hollow rectangular waveguide, this has customarily been done by twisting the guide. Such twisted guides, conventionally, have been made by packing a section of straight waveguide with an incompressible substance, holding the ends of the section in clamps, twisting the guide to the desired extent, and then removing the filling. To rotate the plane of polarization by 90, it has been usual to employ a waveguide section that is several wavelengths long and to twist the section to cause its rota-' tional deformation to be uniformly distributed over substantially the entire length. The gradual twist which occurs over the distance of several wavelengths conduces to a low standing wave ratio.

Twisted sections of waveguide are expensive to fabricate by the conventional process because of the length of the sections, the amount of waste material that results and the number of operations involved in the process. Where the imperfections induced in the rectangular guide by the twisting process exceed permissible limits, the imperfections act as a source of reflections to the wave energy. Twists produced by the conventional process tend to have unreliable mechanical performance because the final dimensions are dependent on the temper of the waveguide material and the electrical performance of such twists tends to be poor because of the discontinuities which appear where the clamps gripped the waveguide ends for twisting. In addition to the difficulty in producing acceptable twisted sections of waveguide, the conventional twist is often inconveniently long for use in transmission systems where it is desired to minimize line lengths.

In US Pat. No. 3,046,507 it was proposed to construct a twisted waveguide from a stack of identical hollow rectangular waveguide segments assembled with each segment turned slightly relative to the preceding contiguous segment. In the patent, each segment of the stack is a flat disc having a centrally disposed rectangular opening and each disc is rotated l relative to the preceding contiguous disc. The interior of that twisted hollow waveguide has an aspect similar to a spiral stairway. That is, in looking directly down into the waveguide, spiraling treads are seen because the transition from one segment to the next forms a step (viz, a tread) that partially obstructs the rectangular opening. To obtain acceptable performance from a twisted waveguide of such stacked segments, a large number of segments must be employed to prevent the steps from materially interferring with the transmission of wave energy. A large number of segments permits a more gradual twist to be obtained but it concurrently necessitates a lengthening of the twisted section.

It should be observed that in the prior art devices, the cross section of the twist is the same at all points along its length; In the formed twist, this cross section turns uniformly along the length of the twist while in the stepped twist a fixed cross section rotates discontinuously at the steps. It will be seen that, in this invention, the cross section rotates by undergoing a continuous deformation in its shape.

OBJECTIVE OF THE INVENTION The purpose of a twisted section of rectangular waveguide is to reorient the plane of polarization of electromagnetic waves transmitted through the section. The primary objective of the invention is to provide a waveguide twist section in which a low standing wave ratio is maintained while the polarization plane of the input signal is rotated through a specified angle in a minimum length of guide. That objective is met by the graded step twist here disclosed which rotates the plane of polarization within a shorter distance than conventional waveguide twists without impairment in performance.

THE INVENTION The invention resides in a hollow waveguide twist constructed of basic graded step sections arranged to form a continuous waveguide. Each basic graded step section is approximately one quarter wavelength long and has inclined broad walls which cause the plane of polarization of the wave energy propagating through the waveguide to be gradually rotated without encountering any abrupt change in cross-sectional area and without storing any appreciable amount of electric or magnetic energy in the section. The inclined walls replace the abrupt steps which occur at the transitions in the waveguide segments of a shear twist. In the preferred embodiment of the invention, the capacitance required to tune out the residual shunt inductance of the twist section is provided by a negative inductive recess at the midsection of basic step section.

THE DRAWINGS The invention, both as to its construction and mode of operation, can be better understood from the exposition which follows when considered in conjunction with the accompanying drawings in which:

FIG. 1A depicts the cross section of a conventional hollow rectangular waveguide;

FIG. 1B shows the cross section of a hollow rectangular waveguide that has been modified to have arcuate end walls;

FIG. 2 depicts a shear twist waveguide employing stacked sections of the modified waveguide;

FIG. 2A represents an equivalent circuit of the FIG. 2 shear twist waveguide;

FIG. 23 represents an equivalent circuit in which the shunt inductances are tuned out by shunt capacitances;

FIG. 3 is a top plan view of the shear twist waveguide;

FIG. 4 depicts the lower two stacked sections of the shear twist waveguide;

FIG. 5 illustrates the development of the basic section of the invention;

FIG. 6 depicts a rudimentary basic section of the invention;

FIGS. 6A to 6G depict the changing cross section of the basic section of FIG. 6;

FIG. 6H is a view of the preferred form of the basic section of the invention;

FIG. 7 depicts three basic sections assembled to form a waveguide twist; and

FIG. 8 illustrates the development of a modified form of the basic section of the invention.

THE EXPOSITION Conventional hollow rectangular waveguide in accordance with standard notation, has its internal width denoted by a and its internal height denoted by b, as in FIG. 1A. To facilitate fabrication of the invention, it is convenient to modify the conventional hollow rectangular waveguide, as in FIG. 13, by making the shorter side arcs of a circle of radius r having its center on the longitudinal axis of the guide and to use that modified configuration as the input and output ports of the invention. Since the waveguide twist is intended to be joined to conventional rectangular waveguide, to reduce the reflection of waves entering the twist component, the height b of the input port of the twist (FIG. 1B) is the same as that of the rectangular waveguide. Further, the area of the input port is preferably identical with the cross-sectional area (a X b) of the rectangular guide. Knowing the height b and the area of the rectangular waveguide opening, the radius r of the circle for the shorter sides of the modified waveguide can be determined by several mathematical procedures to obtain the identical area.

The invention is here developed from a hollow waveguide which has been modified to make its shorter sides arcs of a circle of radius r, as in FIG. 18. Further, for ease of exposition and illustration of the invention, the waveguide walls are deemed to be infinitely thin, though having the electrical conductivity of ordinary metallic waveguide. FIG. 2 shows the modified hollow waveguide divided into four sections 1, 2, 3 and 4, each section being a straight guide member that is approximately Ag/4 in length, where Ag is the average wavelength in the guide for the band of frequencies to be accommodated by the twist. )tg is given by the expression Ag =2Ag, Ag /(Ag hg where Ag is the wavelength in the guide at the lower limit of the band Ag is the wavelength in the guide at the upper limit of the band.

For purpose of exposition, sections 2, 3, and 4 are assumed to have been rotated in the same direction about the longitudinal axis L to cause each section to be at an angle relative to the contiguous section. The angle 0 is here assumed to be an angle of 30 whereby the section 4 is twisted 90 relative to section 1, as indicated in the plan view of FIG. 3. The structure depicted in FIG. 2 is termed a shear twist.

The amount of rotation of the polarization plane provided by a section of the shear twist is governed by the angle 0. In the shear twist configuration, each abrupt transition which results from the large angle 0 between contiguous sections, causes an appreciable part of the wave energy impinging on the transition to be reflected. It has been determined that this reflective discontinuity behaves as if the shear twist consisted of a section of uniform waveguide with a shunt inductance at its midpoint in the location corresponding to the abrupt transition and that the magnitude of the shunt inductance is proportional to the angle of twist 0. For purposes of analysis, the shear twist can be schematically represented, as in FIG. 2A, by a straight section of uniform waveguide having shunt inductances L1, L2, L3 spaced at Ag/4 intervals along the guide. In the invention, which is, an improvement upon the shear twist, the shunt inductance of each section is tuned out at mid-band by shunt capacitances C1, C2, C3, as indicated in FIG. 2B, and the geometry of the invention minimizes the loaded Q of the resonant circuits. 7

To maintain the length of the shear twist of FIG. 2 as short as possible, the fewest number of guide sections are used which provide for overlap of the narrow walls around the angle of twist. That is, referring to FIG. 3 by way of example, the arcuate walls of section 1 overlap the arcuate walls of section 2, the arcuate walls of section 3 overlap the arcuate walls of section 2, and the arcuate walls of section 4, in turn, overlap the arcuate walls of section 3. Where the longer twist can be tolerated, to reduce the amount of energy reflected by the twist more sections may be used and the angle 0 may be reduced to provide a more gradual rotation of the polarization plane.

Consider now FIG. 4 in which only the shear twist waveguide sections 1 and 2 are shown. At the transition between the two sections, four abrupt steps S1, S2, S3, and S4 are present which are closed off by electrically conductive plates to confine the wave energy within the waveguide. It is apparent, therefore, that at the transition, the cross-sectional area of the waveguide opening is abruptly reduced. The abrupt transition has the electrical effect of introducing a shunt inductance, as in FIG. 2A, whose magnitude varies as the angle of twist 0. The invention is an improvement upon the shear twist which avoids the abrupt change in the cross-sectional area of the waveguide opening at the transition in the shear twist and in place of each abrupt step provides in incline or graded step.

The basic element of the preferred embodiment of the invention is developed from the shear twist of FIG. 4 by dividing each of the waveguide sections 1 and 2 into two equal parts as indicated by the dashed lines in that figure. Considering only the segments 1A and 2A, shown in FIG. 5, the lower comers of segment 1A are designated A, B, C, D and the upper corners of segment 2A are designated E, F, G and H. The points where the broad walls of the sections 1A and 2A meet are designated M and N. To replace the step S1 with a graded step, an inclined plane is passed through the points AME. The plane AME intersects the broad wall of the lower section 1A to form the trace AM and intercepts the upper section 2A to form the trace EM on its broad wall. Considering the two segments 1A and 2A to be bounded by a right circular cylinder 10 of radius r which is indicated in phantom in FIG. 5, the plane AME forms the trace AB on the cylinder wall. The portion of the plane AME bounded by the foregoing traces then becomes an inclined wall of the waveguide twist in place of the abrupt step S1. The comer EJMK is thus lopped off from segment 2A by the inclined wall while the comer AJLM is added to segment 1A by that inclined wall.

In similar fashion, step S2 is replaced by an inclined wall bounded by the traces of a plane passing through the points BMF, step S3 is replaced by an inclined wall bounded by the traces of a plane passing through points CNG, and step S4 is replaced by an inclined wall bounded by the traces of a plane passing through points DNI-I. The basic graded step element of the preferred embodiment, therefore is of the form depicted in FIG. 6. In the basic element, the triangular wall ABM is parallel to triangular wall DCN and the triangular wall EMF is parallel to triangular wall I-ING since those walls remain from the original segments 1A and 2A. The triangular wall AME and wall DNI-I are inclined relative to the remnants of the original walls. The triangular wall GNC and its facing wall BMF are also inclined relative to the remnant walls but their inclination is opposite to that of the slope of walls AME and DNI-l.

FIGS. 6A through 66 show the configuration of successive cross sections taken normal to the longitudinal axis L in the basic section of FIG. 6. The input port ABCD, as indicated in FIG. 6A, is of generally rectangular cross section, although it is recognized that the narrow walls are arcuate rather than straight. Moving upwardly from the input port while taking successive cross sections, the cross section changes from slightly serpentine as in FIG. 63 to the more pronounced serpentine form of FIG. 6C. The center parts P1 and P2 of the serpentine form decrease in length as the successive cross sections approach the midpoints M and N of the basic section. The cross section taken through midpoints M and N is the generally rectangular from shown in FIG. 6D. Moving upwardly, the cross-sectional configuration again becomes serpentine as in FIG. 6E with the center parts Q1 and Q2 increasing in length as in FIG. 6F until at exit port EFGH, the configuration returns to generally rectangular as depicted in FIG. 60.

Wave energy propagating through the basic graded step element encounters the inclined surfaces and, consequently, the plane of polarization of the wave energy is rotated through the angle 0 in passing through the element. The basic elements may be stacked one upon another to form a waveguide twist providing the requisite rotation of the plane or polarization. From the viewpoint of electrical performance, the grading" has the effect of markedly reducing the magnitude of the shunt inductance which appears in the equivalent circuit. Therefore, when that residual inductance is tuned out with a shunt capacitance, as symbolically indicated in FIG. 2B, the resulting resonant circuit has a very low Q. The shunt capacitance can, for example, be provided by employing capacitive screws in the broad walls of the twist to facilitate tuning of the basic sections. Such tuning screws are preferably located at midpoints M or N in the basic section of FIG. 6. Preferably, however, the shunt inductance of the basic section is tuned out, as depicted in FIG. 6H, by centrally located recesses 6 and 7 which add the requisite negative" inductance to the section. The recesses 6 and 7 are sectors of an annulus 8 encircling the basic section and, in effect, increase the distance between the narrow walls of waveguide. For a 30 twist section having recesses as in FIG. 6H, a VSWR of no greater than 1.02 over a full waveguide band was obtained.

In the basic graded step section here illustrated, the angle was, for expository purposes, set at 30 B y assembling three basic sections 10, 11, 12 one upon the other, as shown in FIG. 7, a waveguide structure is formed which rotates the plane of polarization through an angle of 90 in a twist that is approximately 3 g/4 in length. By adding other basic sections, the plane of polarization can be rotated through a larger angle. In actual practice, it was found that better results were attained when each basic section was slightly more than g/4 in length.

The equivalent circuit for the FIG. 7 twist is shown in FIG. 2B. Because the angle of twist is the same for each basic sec tion in the FIG. 7 twist, shunt inductance L1 L2 L3 and, as the resonant frequencies of the turned circuits are the same, LlCl L2C2 L3C3. Of course, circuits of the type shown in FIG. 28, being resonant at the same frequency, are characterized by their shunt inductance which, as previously stated, is proportional to the angle of twist The problem of determining the relative angles of a series of basic sections required to yield, when in cascade, a given total twist is equivalent to finding the relative values of the inductances (L1, L2, L3 in FIG. 2B) when the input VSWR performance is specified over a given frequency band. This problem has been considered in the technical literature by a number of authors. In the paper entitled The Application of a New Class of Equal-Ripple Functions to some Familiar Transmission Line Problems, by Henry J. Riblet, IEEE Transactions on Microwave Theory and Techniques, Volume MTT-l 2, No. 4, July 1964, pp. 415-421, there is given an exact theory for solving a problem which is very closely approximated by the problem associated with the FIG. 2B equivalent circuit. By applying that theory to the design of twists covering the usual waveguide band, for example the band 8.2 KMl-lz to 12.4 KMI-Iz designated for WR- 90 waveguide, it is found that for a three section twist, equal ripple performance is realized when the twist angles are approximately the same for all three sections. For a twist having four sections, however, the two center sections should have the same twist angle and each end section should have about 80 percent of the angle ofa center section. Thus, in the three section, 90 twist, illustrated in FIG. 7, the angle of the basic section was chosen to be 30 as the twist is intended to operate over the full waveguide band.

The power handling capability of the graded step waveguide twist is nearly equal to that of the conventional long waveguide twist and is materially better than that of the shear twist inasmuch as the graded step twist does not present the abrupt constrictions that are present at the transitions in the shear twist.

In the preferred embodiment, a graded step inclined side wall, as explained in connection with FIG. 5, is formed, for example, by the plane AME. Thereby the entire length (approximately g/4) of the basic element is utilized since the plane passes through the corners A and E. The entire length of the basic section need not be employed, as the inclined side wall can, as shown in FIG. 8, be formed by passing a plane through the points RMS. The incline from the vertical of the plane RMS is, of course, less gradual than the incline of plane AME and some impairment in performance of the twist results where the full length of the basic section is not utilized for the sloped walls. However, a useful waveguide twist can be made although somewhat less than the full length of basic section is utilized for the graded step. Although the word plane has been used to describe surface AME and others, it is clear that cumed surfaces could be used to obtain the same results.

Although, in the preferred embodiment, the basic section is approximately one quarter wavelength g/4 long, that length is not an essential requisite of the invention. Where the length of the basic section is materially different from a quarter wavelength the tuned circuits in the equivalent circuit are not spaced at g/4 intervals.

In view of the numerous forms which embodiments of the invention can tal e, it is not intended that the scope of the inventron be restricted to the precise structures and arrangements illustrated in the drawings or described in the exposition. Rather, it is intended that the scope of the invention be delimited by the claims appended hereto and that within that scope be included only those waveguide twists which in essence utilize the invention.

What is claimed is:

I. In a waveguide twist of the type having hollow, substantially rectangular waveguide segments disposed in abutment one upon another, each segment being rotated about the longitudinal axis of the waveguide twist relative to the contiguous abutting segment, and in which the transitions between segments are spaced at intervals of approximately g/4, where g is the average wavelength in the guide for the band of frequencies intended to be accommodated by the twist, the improvement comprising inclined walls constituting portions of the .broad walls of the segments, the inclined walls extending between contiguous segments and replacing the abrupt steps normally occurring at the transition between contiguous segments.

2. The improvement according to claim 1, further including shunt capacitive means forming a resonant circuit with the residual shunt inductance associated with a transition between contiguous segments.

3. The improvement according to claim 2, wherein the shunt capacitive means are opposed recesses in the narrow waveguide walls.

4. The improvement according to claim 1, wherein the inclined walls are substantially triangular and each inclined wall has an apex at a point of intersection between the internal broad walls of contiguous segments.

5. The improvement according to claim 4, wherein the narrow walls of the waveguide twist, in cross section, are

arcs of a circle having its center at the longitudinal axis of segment rotation.

6. The improvement according to claim 5, wherein the arcuate narrow walls of the waveguide twist have opposed internal recesses which provide shunt capacitances that resonate with the residual shunt inductances associated with the transitions between contiguous elements.

7. A waveguide twist comprising basic sections connected in tandem to form a continuous hollow waveguide, each basic section having its ends of generally rectangular cross section and having inclined walls constituting portions of the broad walls of the basic section, the inclined walls being disposed to cause the configuration of successive cross sections taken normal to the longitudinal axis of the waveguide twist to gradually change, from generally rectangular to generally serpentine and back to generally rectangular.

8. A waveguide twist comprising a continuous hollow waveguide having input and output ports of generally rectangular configuration, the waveguide twist having its broad walls constituted by a plurality of planar segments, some of the segments being parallel to the longitudinal axis of the waveguide twist, each parallel to the axis segment being angularly disposed relative to the adjacent parallel to the axis segment of the same broad wall whereby those segments spiral about the longitudinal axis, the remainder of the planar segments being inclined to said longitudinal axis and joining the parallel to the axis segments whereby the configuration of successive cross sections taken normal to the longitudinal axis gradually changes from generally rectangular to serpentine and back to generally rectangular. 

1. In a waveguide twist of the type having hollow, substantially rectangular waveguide segments disposed in abutment one upon another, each segment being rotated about the longitudinal axis of the waveguide twist relative to the contiguous abutting segment, and in which the transitions between segments are spaced at intervals of approximately lambda g/4, where lambda g is the average wavelength in the guide for the band of frequencies intended to be accommodated by the twist, the improvement comprising inclined walls constituting portions of the broad walls of the segments, the inclined walls extending between contiguous segments and replacing the abrupt steps normally occurring at the transition between contiguous segments.
 2. The improvement according to claim 1, further including shunt capacitive means forming a resonant circuit with the residual shunt inductance associated with a transition between contiguous segments.
 3. The improvement according to claim 2, wherein the shunt capacitive means are opposed recesses In the narrow waveguide walls.
 4. The improvement according to claim 1, wherein the inclined walls are substantially triangular and each inclined wall has an apex at a point of intersection between the internal broad walls of contiguous segments.
 5. The improvement according to claim 4, wherein the narrow walls of the waveguide twist, in cross section, are arcs of a circle having its center at the longitudinal axis of segment rotation.
 6. The improvement according to claim 5, wherein the arcuate narrow walls of the waveguide twist have opposed internal recesses which provide shunt capacitances that resonate with the residual shunt inductances associated with the transitions between contiguous elements.
 7. A waveguide twist comprising basic sections connected in tandem to form a continuous hollow waveguide, each basic section having its ends of generally rectangular cross section and having inclined walls constituting portions of the broad walls of the basic section, the inclined walls being disposed to cause the configuration of successive cross sections taken normal to the longitudinal axis of the waveguide twist to gradually change, from generally rectangular to generally serpentine and back to generally rectangular.
 8. A waveguide twist comprising a continuous hollow waveguide having input and output ports of generally rectangular configuration, the waveguide twist having its broad walls constituted by a plurality of planar segments, some of the segments being parallel to the longitudinal axis of the waveguide twist, each parallel to the axis segment being angularly disposed relative to the adjacent parallel to the axis segment of the same broad wall whereby those segments spiral about the longitudinal axis, the remainder of the planar segments being inclined to said longitudinal axis and joining the parallel to the axis segments whereby the configuration of successive cross sections taken normal to the longitudinal axis gradually changes from generally rectangular to serpentine and back to generally rectangular. 